The bore of a typical hydraulic cylinder gland has a seal housing portion in which one or more seal rings are seated in annular grooves extending around the bore. The seals are retained axially by the radial walls of the grooves and fit with radial interference between the seal housing bore and the piston rod which acts in it. Usually there is a main rod or gland seal which seals against the piston rod and serves to prevent, or at least to minimise, escape of the hydraulic fluid. It is common also to provide a buffer seal ring, on the pressure side of the rod seal. The purpose of the buffer seal is to shield the rod seal from the direct effect of pressure shock or "spikes" which may occur from time to time. For example, such shocks may occur when the bucket of a hydraulically-powered mechanical digger hits a rock instead of earth.
To achieve this buffering effect, the buffer seal has conventionally been provided as a rectangular-section ring having a degree of axial freedom in its seating groove.
In some instances the sealing of the buffer seal can lead to pressure trapping in the space between the rod seal and the buffer seal. Such pressure trapping may lead to higher friction and wear and, if the trapped pressure is sufficiently high, to extrusion damage. Extrusion damage may be caused to the buffer seal or, more seriously, may affect the rod seal itself.